Conventional phosphate fertilizers are produced by chemical methods, that is, by decomposition of phosphate rock using concentrated sulfuric acid to produce phosphate fertilizers of high solubility. Sulfides such as sulfur dioxide, generated in the process of manufacturing the phosphate fertilizers, release to the atmosphere and lead to air pollution, sulfates originated from the use of phosphate fertilizers causes soil acidification, and 70% or more of phosphate ions are immobilized by elements like aluminium, iron, calcium and magnesium in soil, resulting in soil salinization. Typically, less than 30% of phosphorus in soil is substantively used by crops (plants) indeed.
The grade of phosphate rock is classified by its content (in percentage) of P2O5. Generally speaking, phosphate rock having more than 28% P2O5 is classified as medium to high grade; phosphate rock having 8% to 28% P2O5 is classified as medium to low grade. Currently, medium to high graded phosphate rock is used as the raw material for manufacturing phosphate fertilizers by chemical method, while the cost for processing in the manufacture of the chemical phosphate fertilizers using medium to low graded phosphate rock is too high and thus medium to low graded phosphate rock is not suitable for such preparation. Taking phosphate rock having 32% P2O5 as an example, the phosphorus content contained in the chemical phosphate fertilizers produced by the decomposition of phosphate rock using concentrated sulfuric acid is about 16%. If low graded phosphate rock is used, the cost for raw materials such as phosphate rock and concentrated sulfuric acid as well as the cost for electricity consumption during the manufacture of phosphate fertilizers would be greatly increased.
Medium to low graded phosphate rock constitutes a considerable proportion of the resources for phosphate rock. In order to take full advantage of the resources for phosphate rock, to minimize the waste of the resources and to relieve the contradiction between supply and demand of the phosphate fertilizers, full utilization of the medium to low graded phosphate rock which is not suitable for preparing phosphate fertilizers is of significant importance.
There are many problems with the conventional use of phosphate rock in the manufacture of chemical phosphate fertilizers, which includes: 1) liquid waste of sulfur dioxide and strong acids caused by the plenty use of strong acids for the decomposition of ground phosphate rock in the manufacture of phosphate fertilizers leads to pollutions of air, soil and water sources; 2) utilization of the resources for phosphate rock is relatively low, and that waste of resources is particularly acute on the premise of the current low production yield of the high graded phosphate rock; 3) the use of traditional phosphate fertilizers, to a great extent, causes soil acidification, which significantly constrains the sustainable development of agriculture; 4) the use of high concentrated superphosphate typed fertilizers leads to the high nitrate content in crops, from which the health of people would be suffered and their life safety would render uncertain.
Along with the development of economy and the rise of consumption standard, the desire for foods has been generally changed from quantity fed to safety and quality, and there is growing concern about food quality and its safety issues. Food (especially agricultural products) safety issues have caused wide public concern all over the world. To solve the food safety issues, the root causes should be addressed. As most of the foods for human consumption come from agricultural products, solving the problem of soil pollution, restoring the ecological environment of soil, research and development of biological phosphate fertilizers represent the important measures with promising future for ensuring food safety.